The first stage of the technical specification 3GPP TS 22.146 V5.0.0 (2001-10): “Multimedia Broadcast/Multicast Service; Stage 1 (Release 5)” specifies requirements for a service called Multicast Broadcast Multimedia Service (MBMS). The MBMS is to enable point-to-multipoint transmissions of multimedia data, like text, audio, picture, or video. For the MBMS, two modes of operation are provided, the broadcast mode and the multicast mode. Unlike the broadcast mode, the multicast mode generally requires a subscription to a multicast subscription group and an activation of the service. In broadcast mode, the multimedia data is transmitted from a single source point via a radio interface to a broadcast area, while in multicast mode, the multimedia data is transmitted from a single source point via a radio interface to a multicast group in a multicast area. In both modes, a plurality of mobile terminals may thus receive the same data over a common radio channel, which enables an efficient use of radio resources. The realization of the MBMS over the radio access network has not been specified yet
For MBMS data that is to be transmitted in a radio cell of a cellular communication network, a transport mechanism is required, which takes into account the general capacity allocation in this radio cell.
Transmissions to and from GERAN are based on time division multiple access (TDMA). In a TDMA system, the time on the radio interface is divided in to frames, and each frame is further divided into several time slots. In the case of GSM/GPRS (General Packet Radio Service), there are 8 time slots per frame, as indicated in FIG. 1.
For the transmission of data via the radio interface, a physical channel is defined as a sequence of frames on a time slot and frequency. For illustration, FIG. 2 shows for a single radio frequency carrier the sequence of time slots 21 for 8 consecutive TDMA frames arranged on a helix such that the same time slot in the different frames appears at the same angular location on the helix. A physical channel using only this carrier is composed by a sequence of the respectively same time slot 22 in these frames. The time slot 22 in each TDMA frame belonging to the represented physical channel is pointed out in the figure by shading. The physical content of a time slot, i.e. the corresponding period of the radio frequency carrier which is modulated by a data stream, is referred to as burst.
In GPRS and EGPRS (Enhanced GPRS), a physical channel is further divided into blocks of four consecutive frames, which is equally indicated in FIG. 2. Data can be transferred with this structure in four block-interleaved bursts, while there is one burst per time slot and per frame. The blocks can carry a varying amount of information, depending on the modulation and coding scheme used. Information from several logical channels, e.g. Packet Data Channel (PDCH), Packet Common Control Channel (PCCCH), Packet Associated Control Channel (PACCH), etc. can be multiplexed into the different blocks of one physical channel. Thus, a first block can be associated to a PDCH, the next block to a PCCCE, and so on. It is also possible to multiplex several users into a single PDCH.
In order to support a specific data rate for a specific service, the network is further able to allocate for a service several physical channels in parallel and all or part of the radio blocks of these physical channels. This way, the network can provide required bit rates in a flexible manner.
The physical channels must be located in the frequency and time domain in a way that reception and transmission by the mobile terminals in the cell is enabled. In case the mobile terminals in the cell are to follow several channels at the same time, multislot class constraints of the mobile terminals have moreover to be taken into account. A mobile terminal can receive and transmit on a predetermined number of time slots, which number depends on the capabilities of the mobile terminal. The number of time slots that are supported for transmission and reception determine the multislot class of the mobile terminal. Typically, a mobile terminal is able to support e.g. 2, 3 or 4 time slots in the downlink and 1 or 2 time slots in the uplink. In the simplest case, the mobile terminal is a 1+1 terminal, which can receive and transmit on only one time slot per direction. The uplink transmission periods and the downlink reception periods allocated to a terminal must be set in such a way that they leave sufficient time in between, in order to enable the mobile terminal to change from reception to transmission and vice versa, to change the frequency, and to carry out neighbor cell measurements. An exemplary allocation taking into account these requirements is illustrated in FIG. 3.
FIG. 3 shows in a first row 19 downlink time slots and in a second row 19 uplink time slots of consecutive TDMA frames. The time slots used by a multislot class 4 mobile terminal in the downlink and in the uplink are pointed out in FIG. 3 by shading. As can be seen, the mobile terminal is capable of receiving on 3 consecutive time slots 3-5 in the downlink and of transmitting on one time slot 4 in the uplink. Due to the offset in the time slot numbering in the downlink and in the uplink, there is always a spare time of at least one time slot for changing from reception to transmission and vice versa.
A PDCH is a physical channel allocated to carry packet logical channels. In an idle mode, a mobile terminal is prepared to transfer packet data units (PDU) on PDCHs, but no radio resources on a PDCH are allocated for the mobile terminal. During the idle mode, the terminal has to be able, however, to follow a packet broadcast control channel (PBCCH) and its paging group on the PCCCH.
A broadcast control channel is a point-to-multipoint uni-directional control channel from the fixed sub-system to the mobile terminals. The PBCCH constitutes one of these broadcast control channels and broadcasts parameters used by the mobile terminals to access the network for packet transmission operation. The PBCCH is mapped onto one PDCH only, and thus only on one time slot. One of a plurality of parameters contained in the PBCCH is the parameter BS_PBCCH_BLKS, which indicates the number of blocks allocated to the PBCCH in a TDMA multiframe.
The PCCCH is one of several common control channels constituting a point-to-multipoint bi-directional control channel. The PCCCH and its different logical channels, i.e. the Packet Paging Channel (PPCH), the Packet Notification Channel (PNCH), the Packet Random Access Channel (PRACH) and the Packet Access Grant Channel (PAGCH), can be mapped dynamically to physical channels. Except for the PPCH and the PRACH, the PCCCH can be mapped on all blocks except for those used for the PBCCH. A terminal registered to a network is allocated to a specific paging group, and paging messages are transmitted by the PPCH associated to the respective paging group. A paging message comprises data which is required when the network wants to initiate a communication with the terminal.
If there is only one PCCCH in the cell, it is on the same time slot as the PBCCH. If there is a need for additional PCCCHs, these can be assigned to other time slots and/or to other carriers. In case of multiple PCCCHs, the blocks used according to the BS_PBCCH_BLKS parameter for PBCCH on the shared PBCCH/PCCCE time slot are not used for other PCCCHs in additional PCCCH time slots. These blocks can only be used in addition for packet data traffic channels (PDTCH). Thereby, the mobile terminals are always able to receive the PBCCH and the paging block also in a multiple PCCCH case, without the risk of loosing either PBCCH information or a paging message